In a pot off the heat, combine the brown sugar and cornstarch. Whisk in the egg yolks and half of the milk (1/2 cup/125 ml) until smooth. Whisk in the remaining milk and the cream. Bring to a boil over medium heat, whisking constantly and scraping the sides and bottom of the pan until the mixture has thickened. Simmer for 1 minute, whisking constantly. Remove from the heat. Whisk in the bourbon and vanilla. Strain through a sieve. Stir in the butter.